1. Field of the Invention
This invention relates to a motive force (power) control system for a vehicle equipped with an internal combustion engine and an automatic transmission, particularly to a vehicle motive force control system that optimizes vehicle fuel economy by integrated control of the engine and the automatic transmission.
2. Description of the Prior Art
Japanese Laid-Open Patent Application No. Hei 6(1994)-294,464 teaches a vehicle motive force control system. This prior art system uses the output speed of an automatic transmission (a continuously variable transmission or CVT), i.e., the rotational speed on the driven side of the transmission, which is proportional to the vehicle speed, and the accelerator pedal position as parameters for determining the desired the CVT output (driven side) torque, i.e., the output torque for optimum fuel economy, by retrieval from predefined (mapped) characteristics or by calculation.
The system further divides the retrieved desired CVT output (driven side) torque by the torque transmission ratio to calculate the desired CVT input torque, i.e., the desired engine output (torque). Next it uses the calculated desired engine output (torque) and the engine speed as address data for retrieving the desired quantity of fuel injection from another map (mapped data). The retrieved desired quantity of fuel injection and the engine speed are then used as address data for retrieving the desired throttle valve opening from a third map and the throttle valve is controlled to the desired opening by operating an actuator.
Japanese Laid-Open Patent Application No. Sho 61(1986)-119,856 (Patent Publication No. Hei 5(1993)-262,263) teaches a system for improving vehicle fuel economy by using the accelerator (pedal) position and vehicle speed as address data for retrieving the desired motive force (basic torque) from a prescribed map, correcting the desired motive force based on change in accelerator position etc., and determining the gear or speed ratio and the quantity of fuel injection (corrected) based on the corrected desired motive force.
These conventional systems are, however, not completely compatible with the various control systems and methods developed in recent years for improving fuel economy, which involve techniques such as exhaust gas recirculation (EGR) control in which a portion of the exhaust gas is recirculated to the intake system and lean-burn control in which the air/fuel ratio is regulated to be leaner than the stoichiometric air/fuel ratio. For example, these conventional systems are made ineffective by the fact that the optimum combination of engine torque and gear (speed) ratio relative to the same desired motive force does not necessarily remain the same when the control mode is changed from stoichiometric air/-fuel ratio control to the lean-burn control.
The conventional systems are based on the assumption that the engine is always controlled in the mode that provides the best fuel economy. They therefore invariably determine the same desired gear (speed) ratio for any given desired motive force (power). Because of this, they cannot always achieve the expected optimum fuel economy when the engine control mode is changed.
In other words, depending on the states or condition of the engine operation such as during warmup or learning control or due to changes in the environment external to the engine, it sometimes happens that the engine cannot be controlled in the desired mode. The control may also be restricted owing to emission considerations.
At any rate, the conventional systems do not determine the gear (speed) ratio in accord with the actual engine operating states and condition. They are therefore incapable of constantly maintaining optimum fuel economy irrespective of the actual engine operating states and condition.